Vehicle headway maintenance assist system and method

ABSTRACT

A vehicle headway maintenance assist system is provided that provides a haptic notification to an accelerator to alert the driver under prescribed conditions. The system changes a drive force/accelerator actuation relationship between an accelerator actuation amount and a target drive force or torque under the prescribed conditions. In particular, system changes a first drive force/accelerator actuation relationship to a second drive force/accelerator actuation relationship based on a vehicle running condition that was detected so that the driver more readily notices the haptic notification (e.g., an accelerator actuation reaction force) when the haptic notification is being applied to the accelerator to alert the driver under the prescribed conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application Nos.2006-066584, filed on Mar. 10, 2006 and 2007-010208, filed on Jan. 19,2007. The entire disclosures of Japanese Patent Application Nos.2006-066584 and 2007-010208 are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an assistance technique formaintaining a headway distance between a host vehicle and a precedingvehicle. More specifically, the present invention relates to maintaininga headway distance from a preceding vehicle based on a running conditionof the host vehicle and providing a haptic signal in the accelerator.

2. Background Information

Vehicle assist systems have been proposed to alert a driver inaccordance with a reduction in a headway distance, and assist in themaintenance of headway distance. In Japanese Laid-Open PatentApplication No. 2005-8147, it has been proposed to alert the driver ofthe reduction in the headway distance by increasing the reaction forceof the accelerator in accordance with a reduction in headway distance.

In view of the conventional headway maintenance assist systems, it willbe apparent to those skilled in the art from this disclosure that thereexists a need for an improved vehicle headway maintenance assist system.This invention addresses this need in the art as well as other needs,which will become apparent to those skilled in the art from thisdisclosure.

SUMMARY OF THE INVENTION

However, it has been discovered that when the accelerator position is ata low setting such as when the vehicle speed is low, i.e., when theaccelerator is being lightly pressed, the driver does not easilyrecognize when a reaction force is being applied to the accelerator.Thus, the driver may not be alerted of the reduction in the headwaydistance to a preceding vehicle.

In accordance with one aspect of the present invention, a vehicleheadway maintenance assist system is provided that basically comprises apreceding vehicle detection section, an accelerator actuation amountdetection section, a reaction force computing section, a reaction forcegenerating section, a running condition detection section, a drivingforce determination section, a drive source control section and acompensation section. The preceding vehicle detection section isconfigured to detect a headway distance between a host vehicle and apreceding vehicle. The accelerator actuation amount detection section isconfigured to detect an accelerator actuation amount of an acceleratorof the host vehicle. The reaction force computing section is configuredto calculate a reaction force to be generated in the accelerator basedon the headway distance detected by the preceding vehicle detectionsection. The reaction force generating section is configured to generatethe reaction force calculated by the reaction force computing section inthe accelerator. The running condition detection section is configuredto detect a running condition of the host vehicle. The driving forcedetermination section is configured to determine a target driving forceof the host vehicle in accordance with the accelerator actuation amountdetected by the accelerator actuation amount detection section. Thedrive source control section is configured to control an output of adrive source towards the driving force determined by the driving forcedetermination section. The compensation section is configured toincrease a driver awareness of the reaction force generated in theaccelerator by changing from a first drive force/accelerator actuationrelationship between the target drive force determined by the drivingforce determination section and the accelerator actuation amountdetected by the accelerator actuation amount detection section to asecond drive force/accelerator actuation relationship based on therunning condition detected by the running condition detection section.

These and other objects, features, aspects and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed description, which, taken in conjunction with theannexed drawings, discloses a preferred embodiment of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a vehicle is schematically illustrated with a vehicle headwaymaintenance assist system in accordance with a first embodiment of thepresent invention;

FIG. 2 is a flowchart showing the processing executed by the headwaymaintenance assist system in accordance with the first embodiment of thepresent invention;

FIG. 3 is a flowchart showing the processing executed for calculatingthe first headway distance threshold L*1 in the headway maintenanceassist system of the first embodiment of the present invention;

FIG. 4 is a flowchart showing the processing executed for calculatingthe second headway distance threshold L*2 in the headway maintenanceassist system of the first embodiment of the present invention

FIG. 5 is a flowchart showing the processing executed for controllingthe reaction force that is imparted to the accelerator pedal in theheadway maintenance assist system of the first embodiment;

FIG. 6 is a diagram showing the relationship between the gain Kr2 andthe acceleration/deceleration speed αa of the preceding vehicle;

FIG. 7 is a flowchart showing the processing executed for controllingthe changes in the relationship between the target drive force and theposition of the accelerator pedal;

FIG. 8 is a diagram showing the relationship between the vehicle speed Vand the accelerator pedal position Acc;

FIG. 9 is a diagram showing the relationship between the acceleratorpedal position Acc and the target drive force τ*t after a change in thecorrespondence relationship;

FIG. 10 is a diagram showing another example of the relationship betweenthe accelerator pedal position Acc and the target drive force τ*t;

FIG. 11 is a diagram showing an example of the relationship between thespeed V of the host vehicle and the headway distance threshold parameter(i.e., the upper limit Ta_max);

FIG. 12 is a diagram showing an example of the relationship between theaccelerator pedal position Acc and the target drive force τ*t thatillustrates the relationship between the speed V of the host vehicle andthe upper limit Ta_max;

FIG. 13 is a diagram showing the headway distance threshold L* when thedriver ceases to operate (depress) the accelerator pedal; and

FIG. 14 is a diagram showing the relationship between the relative speedVr and the gain Kr.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Selected embodiments of the present invention will now be explained withreference to the drawings. It will be apparent to those skilled in theart from this disclosure that the following descriptions of theembodiments of the present invention are provided for illustration onlyand not for the purpose of limiting the invention as defined by theappended claims and their equivalents.

Referring initially to FIG. 1, a vehicle (hereinafter also called “thehost vehicle”) is schematically illustrated with a vehicle headwaymaintenance assist system in accordance with a first embodiment of thepresent invention. In this illustrated embodiment, the headwaymaintenance assist system is installed in the host vehicle that is arear-wheel drive vehicle having an automatic transmission and aconventional differential gear. The host vehicle includes a hydraulicbraking apparatus that uses a brake pedal 1, a booster 2, a mastercylinder 3, a reservoir 4 and a pressure control unit 5 for the driverto input a target braking force to a left front wheel 10 via a brakedisk 11 and a wheel cylinders 12, a right front wheel 20 via a brakedisk 21 and a wheel cylinders 22, a left rear wheel 30 via a brake disk31 and a wheel cylinders 32, and a right rear wheel 40 via a brake disk41 and a wheel cylinders 42. In this host vehicle, the front wheels 10and 20 and the rear wheels 30 and 40 are configured so that the brakingforce of the left and right wheels can be independently controlled.Thus, the brake disks 11, 21, 31 and 41 and the wheel cylinders 12, 22,32 and 42 are configured and arranged in a conventional manner such thatthe wheel cylinders 12, 22, 32 and 42 frictional hold a correspondingone of the brake disk disks 11, 21, 31 and 41 for imparting a brakeforce (braking force) to each wheel by supplying hydraulic brakingpressure to each of the wheel cylinders 12, 22, 32 and 42 of the wheels10, 20, 30 and 40.

The pressure control unit 5 is disposed between the master cylinder 3and the wheel cylinders 12, 22, 32 and 42. Hydraulic pressure that hasbeen increased by the master cylinder 3 is supplied to each of the wheelcylinders 12, 22, 32 and 42 in accordance with the amount by which thedriver depresses the brake pedal 1. The pressure control unit 5individually controls the brake fluid pressure of the wheel cylinders12, 22, 32 and 42. The pressure control unit 5 has actuators for formingseparate front, rear, left, and right hydraulic pressure supply systems(channels). Thus, the wheels 10, 20, 30 and 40 are thereby separatelybraked. The actuators are configured using proportion solenoid valves sothat, e.g., the hydraulic pressure of the wheel cylinders 12, 22, 32 and42 can be set to an arbitrary brake fluid pressure.

The host vehicle includes an engine 6, a throttle control device 7, anautomatic transmission 8 and a steering wheel 9 as well as otherconventional power train components. The host vehicle also includes adriving/braking force controller 50 and a drive force controller 60. Thedrive force controller 60 is configured to control a drive force(driving force) of the rear (drive) wheels 30 and 40 based on a driveforce instruction value inputted from the driving/braking forcecontroller 50. The driving/braking force controller 50 is configured toperform engine control by controlling an amount of fuel injected intothe engine 6, for controlling the throttle position with the throttlecontrol device 7, for controlling the automatic transmission 8, as wellas other conventional components relating the engine 6. Thus, the driveforce of the rear (drive) wheels 30 and 40 is based on this control ofthe engine 6.

The host vehicle further includes a plurality of wheel speed sensors 13,23, 33 and 43, a steering angle sensor 52, an acceleration sensor 53, ayaw rate sensor 54, a master cylinder fluid pressure sensor 55 and anaccelerator pedal position sensor 56. The signals from the sensors 13,23, 33, 43, and 52 to 56 are inputted to the driving/braking forcecontroller 50. In particular, the wheel speed sensors 13, 23, 33 and 43constitute a wheel speed detection section that is configured andarranged to detect the rotational wheel speeds Vw1, Vw2, Vw3, Vw4 of thewheels and send a signal indicative of the wheel speeds to thedriving/braking force controller 50. The steering angle sensor 52constitutes a steering angle detection section that is configured andarranged to detect a steering angle δ of the steering wheel 9 and send asignal indicative of the steering angle to the driving/braking forcecontroller 50. The acceleration sensor 53 constitutes an accelerationdetection section that is configured and arranged to detect thelongitudinal acceleration Xg of the vehicle and the transverseacceleration Yg of the vehicle and send a signal indicative of thelongitudinal and transverse accelerations of the vehicle to thedriving/braking force controller 50. The yaw rate sensor 54 constitutesa yaw rate detection section that is configured and arranged to detectthe yaw rate øgenerated in the vehicle and send a signal indicative ofthe yaw rate to the driving/braking force controller 50. The mastercylinder fluid pressure sensor 55 constitutes a master cylinder fluidpressure detection section that is configured and arranged to detect themaster cylinder fluid pressure Pm and send a signal indicative of themaster cylinder fluid pressure to the driving/braking force controller50. The accelerator pedal position sensor 56 constitutes an acceleratorpedal position detection section that is configured and arranged todetect the accelerator pedal position and send a signal indicative ofthe accelerator pedal position to the driving/braking force controller50.

The drive force controller 60 is configured to calculate an enginetorque τa, a desired drive force or torque τm based on the amount bywhich the accelerator pedal 1 is depressed by the driver, and a driveforce or torque τw in the drive wheel shaft. The engine torque τa, thedesired drive force τm and the drive force τw are inputted from thedrive force controller 60 to the driving/braking force controller 50.

The host vehicle further includes a laser radar 70, which for example ismounted in a front portion of the host vehicle such as in the frontgrill, the front bumper or in some other appropriate location of thevehicle. The laser radar 70 detects the headway distance L from thepreceding vehicle and the relative speed Vr by sending laser light outin front of the host vehicle and receiving the light that is reflectedback off the preceding vehicle located in front of the host vehicle. Therelative speed Vr is a value obtained by subtracting the speed of thepreceding vehicle from the speed of the host vehicle. The headwaydistance L and the relative speed Vr detected by the laser radar 70 aresent to the driving/braking force controller 50. Thus, the laser radar70 constitutes a preceding vehicle detection section that is configuredand arranged to detect the headway distance L and the relative speed Vr,and the send a signal indicative of the headway distance and therelative speed to the driving/braking force controller 50.

The host vehicle further includes an accelerator pedal actuator 80 andan accelerator pedal 81. The accelerator pedal actuator 80 is configuredand arranged to impart a reaction force to the accelerator pedal 81based on a command from the driving/braking force controller 50. As usedherein, the term “reaction force” refers to force that is applied in adirection opposite to the direction in which the driver depresses theaccelerator pedal 81. Thus, the accelerator pedal actuator 80constitutes a haptic information conveying section that is configuredand arranged to convey a risk potential to a driver as hapticinformation through the accelerator pedal 81, which constitutes adriver-operated driving operation device.

In the headway maintenance assist system in the illustrated embodimentof the present invention, when the headway distance L between the hostvehicle and the preceding vehicle is less than a first headway distancethreshold L*1, a headway maintenance assistance control is performed inaccordance with the operation of the accelerator pedal 81. Inparticular, when the driver is not operating (depressing) theacceleration pedal 81, the headway maintenance assistance controlincludes a primary deceleration control that is performed to deceleratethe vehicle when the headway distance L is less than the first headwaydistance threshold L*1 and a secondary deceleration control that isperformed to decelerate the vehicle when the headway distance L is lessthan a second headway distance threshold L*2, which is less than thefirst headway distance threshold L*1. However, if the driver isoperating the accelerator pedal 81 when the headway distance L is lessthan the second headway distance threshold L*2 (L*2<L*1), then areaction force is applied to the accelerator pedal 81.

According to the present invention, it is possible to positively notifythe driver that the reaction force has been applied to the acceleratorpedal 81 because the relationship between the drive force of the hostvehicle and the accelerator pedal position is corrected based on thedetected running condition of the host vehicle so that the driver canmore readily notice an accelerator actuation reaction force. In otherwords, in the present invention, the running condition of the hostvehicle and the accelerator pedal actuation amount are detected, and arelationship between the target drive force of the host vehicle and apredetermined accelerator pedal actuation amount is corrected based onthe running condition of the host vehicle. However, an accelerationcontrol corresponding to the preceding vehicle is not necessarilyperformed when the preceding vehicle accelerates. Thus, the headwaymaintenance assist system as described herein is not equipped with anacceleration control program to maintain a prescribed following distancefrom the preceding vehicle (e.g., an adaptive cruise control). Ofcourse, an adaptive cruise control could be included if needed and/ordesired.

The detailed processes of the headway maintenance assist system will nowbe described with reference to FIGS. 2 through 10.

FIG. 2 is a flowchart showing the process performed by the headwaymaintenance assist system in the illustrated embodiment. When the hostvehicle is started up, the driving/braking force controller 50 initiatesthe process step S400. In step S400, the following data is read, e.g.,the accelerator pedal position Acc detected by the accelerator pedalposition sensor 56, the wheel speeds Vw1, Vw2, Vw3, Vw4 detected by thewheel speed sensors 13, 23, 33 and 43, and the headway distance L andthe relative speed Vr with respect to the preceding vehicle as detectedby the laser radar 70. The process then advances to step S401.

In step S401, the driving/braking force controller 50 executes a torqueor force characteristic correction control that makes adjustments to theforce characteristic by changing the relationship between theaccelerator pedal 81 and the drive force in accordance with the currentrunning condition of the host vehicle. The specifics of the routine forcontrolling changes in the relationship between the accelerator pedaland the drive force are described in detail later with reference to theflowchart shown in FIG. 7. After the relationship between the positionof the accelerator pedal and the drive force has been changed in stepS401, the process advances to step S410.

In step S410, a first headway distance threshold L*1 is calculated. Thefirst headway distance threshold L*1 is calculated from the sum of asteady-state term L*h1, which does not depend on the running conditionof the host vehicle, and a transient term L*r1, which depends on therunning condition of the host vehicle. The specific method forcalculating the first headway distance threshold L*1 will be describedusing the flowchart shown in FIG. 3.

In step S500 of the flowchart shown in FIG. 3, the steady-state termL*h1 is calculated according to Equation 1 below.L*h1=Va×Th  (Equation 1)

In this Equation 1, the parameter Va represents the speed of thepreceding vehicle as calculated based on the speed V of the host vehicleand the relative speed Vr, while the parameter Tb represents a specificheadway time of the host vehicle. The speed V of the host vehicle iscalculated by determining a mean value of the speeds Vw1 and Vw2 of thefront wheels as detected by the vehicle speed sensors 13 and 23.

In step S510, which follows step S500, the driving/braking forcecontroller 50 determines whether the accelerator pedal position Acedetected by the accelerator pedal position sensor 56 is equal to orgreater than a specific accelerator pedal position threshold Acc0. Ifthe accelerator pedal position Acc is determined to be equal to orgreater than the specific accelerator pedal position threshold Acc0,then it is determined that the driver is depressing on the acceleratorpedal 81. Thus, an accelerator operation flag Face is turned “on” whenit is determined that the driver is depressing on the accelerator pedal81, and then the process advances to step S520. If the accelerator pedalposition Acc is determined to be less than the specific acceleratorpedal position threshold Acc0, then it is determined that the driver isnot depressing on the accelerator pedal 81. Thus, the acceleratoroperation flag Face is turned “off” when it is determined that thedriver is not depressing on the accelerator pedal 81, and then theprocess advances to step S530.

In step S520, Equation 2 is used to calculate a first transient termparameter Tr1 for calculating the transient term L*r1 of the firstheadway distance threshold L*1.Tr1=(L−L*h1)/Vr  (Equation 2)

In this Equation 2, the first transient term parameter Tr1 is the timetaken for the headway distance L to reach the steady-state term L*h1 ofthe first headway distance threshold, assuming that the current relativespeed Vr is maintained. When the parameter Tr1 is calculated, theprocess advances to step S530.

As can be seen from the process in steps S510 and S520, the firsttransient term parameter Tr1 for calculating the transient term L*r1 ofthe first headway distance threshold is calculated (renewed) only whenthe accelerator operation flag Facc is turned on. Therefore, the firsttransient term parameter Tr1 is set according to the actual headwaydistance L when the accelerator pedal 81 is being depressed, and theparameter value that was in effect when the accelerator pedal 81 ceasedto be depressed is maintained when the accelerator pedal 81 is not beingdepressed.

In step S530, the transient term L*r1 of the first headway distancethreshold L*1 is calculated according to Equation 3, and the processadvances to step S540.L*r1=Tr1×Vr  (Equation 3)

In step S540, the first headway distance threshold L*1 is calculated byadding together the steady-state term L*h1 of the first headway distancethreshold calculated in step S500, and the transient term L*r1 of theheadway distance threshold calculated in step S520 (see Equation 4).L*1=L*h1+L*r1  (Equation 4)

When the accelerator pedal 81 is being depressed (when the acceleratoroperation flag Facc is turned on), L*1=L according to Equations 2, 3,and 4. After the first headway distance threshold L*1 is calculated,then the process advances to step S420 in the flowchart shown in FIG. 2.

FIG. 13 is a diagram showing the headway distance threshold L*1 when thedriver ceases to depress on the accelerator pedal 81 (i.e., when theaccelerator operation flag Facc is turned from “on” to “off”). Theheadway distance threshold L*1 is set to the headway distance L at thetime the accelerator pedal 81 ceases to be depressed, as shown in FIG.13.

In step S420, the second headway distance threshold L*2 is calculated.The second headway distance threshold L*2 is calculated from the sum ofa steady-state term L*h2 calculated regardless of whether the precedingvehicle is decelerating or not and a transient term L*r2 calculated(updated) when the preceding vehicle is decelerating. The specificmethod for calculating the second headway distance threshold L*2 willnow be described using the flowchart shown in FIG. 4.

In step S600 in the flowchart shown in FIG. 4, the steady-state termL*h2 is calculated based on the speed V of the host vehicle and therelative speed Vr. The function for calculating the steady-state termL*h2 is provided in advance based on the host vehicle speed V and therelative speed Vr. Thus, the steady-state term L*h2 is calculated bysubstituting the host vehicle speed V and the relative speed Vr intothis function. When the steady-state term L*h2 of the second headwaydistance threshold is calculated, the process advances to step S610.

In step S610, the acceleration/deceleration rate αa of the precedingvehicle is calculated, the process advances to step S620. In step S620,a determination is made as to whether a warning flag Fw, which is set ina later-described step S430 (see FIG. 2), has been turned on. Theprocesses in steps S400 through S480 are repeated, and therefore, thedetermination in step S620 is made in this case based on the state ofthe warning flag Fw set during the preceding process. When the warningflag Fw is determined to be “on”, the process advances to step S660, andwhen the warning flag Fw is determined to be “off”, the process advancesto step S630.

In step S630, the driving/braking force controller 50 determines whetherthe acceleration/deceleration rate αa of the preceding vehicle ascalculated in step S610 is equal to or less than a specificacceleration/deceleration rate α0. The specificacceleration/deceleration rate α0 is a threshold for determining whetherthe preceding vehicle is decelerating or accelerating. Thus, the valuesof rates αa and α0 are both positive during acceleration and bothnegative during deceleration. When the acceleration/deceleration rate αaof the preceding vehicle is determined to be equal to or less than thespecific acceleration/deceleration rate α0, the driving/braking forcecontroller 50 determines that the preceding vehicle is decelerating, apreceding vehicle deceleration flag Fdec_a is turned “on”, and theprocess then advances to step S640. When the acceleration/decelerationrate αa of the preceding vehicle is determined to be greater than thespecific acceleration/deceleration rate α0, the driving/braking forcecontroller 50 determines that the preceding vehicle is not decelerating,the preceding vehicle deceleration flag Fdec_a is turned “off”, and theprocess advances to step S650.

In step S640, a second transient term parameter Tr2 is calculated fromEquation 5, below, for calculating the transient term L*r2 of the secondheadway distance threshold.Tr2=(L−L*h2)/Vr  (Equation 5)

In this Equation 5, the second transient term parameter Tr2 is the timeresulting from dividing the remaining distance (L−L*h2) by the relativespeed Vr. The remaining distance is the actual headway distance L lessthe steady-state term L*h2 of the second headway distance threshold atthe time when the preceding vehicle begins to decelerate. When thesecond transient term parameter Tr2 has been calculated, the processadvances to step S660.

In step S650, which takes effect after it is determined that thepreceding vehicle is not decelerating, the second transient termparameter Tr2 is cleared (i.e., set to 0) for calculating the transientterm L*r2 of the second headway distance threshold, and the processadvances to step S660.

In step S660, the transient term L*r2 of the second headway distancethreshold is calculated from Equation 6, below, and the process advancesto step S670.L*r2=Tr2×Vr  (Equation 6)

In step S670, the second headway distance threshold L*2 is calculated byadding the steady-state term L*h2 and the transient term L*r2 of thesecond headway distance threshold (see Equation 7).L*2=L*h2+L*r2  (Equation 7)

In step S670, when the second headway distance threshold L*2 has beencalculated, the process advances to step S430 in the flowchart shown inFIG. 2. In step S430, the warning flag Fw is set. Therefore, a deviationΔL2 between the second headway distance threshold L*2 calculated in stepS420 and the headway distance L from the preceding vehicle detected bythe laser radar 70 is first calculated using Equation 8, below.ΔL2=L*2−L  (Equation 8)

If the deviation ΔL2 calculated from Equation 8 is equal to or greaterthan 0, the headway distance L from the preceding vehicle is equal to orless than the second headway distance threshold L*2, and the warningflag Fw is therefore turned “on” in step S430. If the deviation ΔL2 isless than 0, the warning flag Fw is turned “off” in step S430. Theprocess then advances to step S440 after the warning flag Fw has beenset.

In step S440, accelerator pedal reaction force control is implemented inwhich the reaction force is applied to the accelerator pedal 81 inaccordance with the deviation ΔL2 in the headway distance. The detailedprocesses of this control accelerator pedal reaction force for applyingreaction force to the accelerator pedal 81 are described using theflowchart shown in FIG. 5.

In step S700 of the flowchart shown in FIG. 5, the target acceleratorpedal reaction force τ*a is calculated from Equation 9.τ*a=Kp×ΔL2  (Equation 9)

The value Kp (Kp>0) in Equation 9 is a specific gain for calculating thetarget accelerator pedal reaction force τ*a from the headway distancedeviation ΔL2.

In step S710, which follows step S700, the accelerator pedal actuator 80is instructed to subject the accelerator pedal 81 to a reaction forcecorresponding to the target accelerator pedal reaction force τ*acalculated in step S700. Having received this instruction, theaccelerator pedal actuator then applies a reaction force correspondingto the target accelerator pedal reaction force τ*a to the acceleratorpedal 81. As is clear from Equation 9, the reaction force is applied tothe accelerator pedal 81 when the headway distance deviation ΔL2 ispositive; i.e., when the headway distance L is less than the headwaydistance threshold L*2. When the process in step S710 is complete, theprocess advances to step S450 in the flowchart shown in FIG. 2.

In step S450, a first target deceleration rate α*1 is calculated fromEquation 10 based on the first headway distance threshold L*1 calculatedin step S410 and based on the headway distance L from the precedingvehicle detected by the laser radar 70.α*1=Kv×Kr1×(L*1−L)  (Equation 10)

The value Kr1 is the gain for calculating the first target decelerationforce produced in the host vehicle. The gain Kv is the gain forconverting the target deceleration force into the target decelerationrate, and is set in advance based on the host vehicle specifications.The first target deceleration rate (α*1 is a positive value duringacceleration and a negative value during deceleration.

FIG. 14 is a diagram showing the relationship between the relative speedVr and the gain Kr1. As shown in FIG. 14, the greater the relative speedVr; i.e., the closer the host vehicle is to the preceding vehicle, thegreater the gain Kr1 is; and the smaller the relative speed Vr is, thesmaller the gain Kr1 is. When the relative speed Vr is less than a firstrelative speed Vr1, then the value of the gain Kr1 is set to a firstspecific gain Kr1 a. When the relative speed Vr is greater than a secondrelative speed Vr2, the value of the gain Kr1 is a second specific gainKr1 b. The table specifying the relationship between relative speed Vrand gain Kr1, as shown in FIG. 14, is stored in advance in the memory(not shown) of the driving/braking force controller 50, and the gain Kr1is determined based on this table and the relative speed Vr.

As described above, when the accelerator pedal 81 is being depressed(when the accelerator operation flag Facc is “on”), the first targetdeceleration rate α*1 is 0 because L*1=L. In cases in which the absolutevalue of the change rate (degree of deceleration) of the first targetdeceleration rate α*1 calculated from Equation 10 is less than aspecific first lower limit Δα*1, the absolute value of the change rateof the first target deceleration rate α*1 is set to the lower limitΔα*1. When the first target deceleration rate α*1 has been calculated,the process advances to step S460.

In step S460, a second target deceleration rate α*2 is calculated fromEquation 11, based on the second headway distance threshold L*2calculated in step S420 and based on the headway distance L from thepreceding vehicle detected by the laser radar 70.α*2=Kv×Kr2×(L*2−L)  (Equation 11)

The value Kr2 is the gain for calculating the second target decelerationforce produced in the host vehicle, and the value of the second targetdeceleration rate α*2 when the accelerator pedal 81 is not beingdepressed (when the accelerator operation flag Facc is “off” and thetarget drive force τ*t is set to 0). The second target deceleration rateα*2 is a positive value during acceleration and a negative value duringdeceleration.

FIG. 6 is a diagram showing the relationship between theacceleration/deceleration rate αa of the preceding vehicle and the gainKr2. As shown in FIG. 6, the lower the acceleration/deceleration rate αaof the preceding vehicle; i.e., the greater the rate of deceleration ofthe preceding vehicle is, the greater of the gain Kr2 is. The greaterthe rate of deceleration of the preceding vehicle is, the greater therate of deceleration of the host vehicle can also be set duringdeceleration braking. The value of the gain Kr2 is set to a specificvalue (e.g., 1) in a region in which the acceleration/deceleration rateαa of the preceding vehicle is greater than a specificacceleration/deceleration rate αa1. A table specifying the relationshipbetween the acceleration/deceleration rate αa of the preceding vehicleand the gain Kr2, as shown in FIG. 6, is stored in advance in the memory(not shown) of the driving/braking force controller 50, and the gain Kr2is determined based on this table and the acceleration/deceleration rateαa of the preceding vehicle.

When the absolute value (degree of deceleration) of the rate of changeof the second target deceleration rate α*2 calculated from Equation 11is greater than a specific second upper limit Δα*2 (Δα*2>α*1), theabsolute value of the rate of change of the second target decelerationrate α*2 is limited so as to be equal to or less than the upper limitΔα*2. Increasing the second upper limit Δα*2 past the first upper limitΔα*1 moderately controls deceleration when the headway distance L isless than the first headway distance threshold L*1. Deceleration can becontrolled to quickly move the vehicle to an appropriate headwaydistance when the headway distance is less than the second headwaydistance threshold L*2 (L*2<L*1). When the second target decelerationrate α*2 is calculated, the process advances to step S470.

In step S470, the final target deceleration rate α* produced in thevehicle is determined. In this step, the first target deceleration rateα*1 calculated in step S450 is compared with the second targetdeceleration rate α*2 calculated in step S460, and the smallerdeceleration rate; i.e., the target deceleration having a greater degreeof deceleration is set as the final target deceleration rate α*. In thiscase as well, the final target deceleration rate α* is a positive valueduring acceleration and a negative value during deceleration.

In step S480, which follows step S470, braking is controlled based onthe final target deceleration rate α*. First, as shown in Equation 12, atarget deceleration rate α*brk produced by the brakes is calculated bysubtracting a deceleration rate α*eng produced by engine braking fromthe final target deceleration rate α* determined in step S470.α*brk=α*−α*eng  (Equation 12)

The values α*, α*brk, and α*eng are all positive during acceleration andnegative during deceleration. When the accelerator pedal 81 is beingdepressed (when the accelerator operation flag Facc is on), α*brk=0because α*=α*eng=0.

Next, a target brake fluid pressure P* is calculated from Equation 13based on the calculated target deceleration rate α*brk.P*=−(Kb×α*brk)  (Equation 13)

The value Kb is the gain for converting the target deceleration rateinto a target brake fluid pressure, and is set in advance based on thehost vehicle specifications. When the accelerator pedal 81 is beingdepressed (when the accelerator operation flag Face is on), P*=0 becauseα*brk=0.

The pressure control unit 5 is then instructed to create a brake fluidpressure based on the calculated target brake fluid pressure P*. Havingreceived this instruction, the pressure control unit 5 creates a brakefluid pressure based on the target brake fluid pressure P*, and suppliesthe brake fluid pressure to the wheel cylinders 12, 22, 32 and 42.Control for decelerating the vehicle is thereby implemented if thedriver is not operating the accelerator pedal 81 when the headwaydistance L is less than both the first headway distance threshold L*1and the second headway distance threshold L*2. When the driver isoperating the accelerator pedal 81, deceleration control is notimplemented because the target brake fluid pressure P*=0.

Upon being completed in step S480, the process returns to step S400. Theprocesses in steps S400 through S480 are thereafter repeated.

The process executed by the driving/braking force controller 50 in stepS401 will now be described. In step S401, the torque or forcecharacteristic correction control is implemented for changing therelationship between the actuation amount of the accelerator pedal 81and the drive force. The processing for changing the relationshipbetween the actuation amount of the accelerator pedal and the driveforce will be described with reference to the flowchart shown in FIG. 7.

In step S820 of the flowchart shown in FIG. 2, the driving/braking forcecontroller 50 determines whether the drive force control flag Ft is setto 1. The drive force control flag Ft is set to 1 when a precedingvehicle is detected by the laser radar 70 and in other situations, andthe flag is set to 0 when such prescribed conditions have not besatisfied. The prescribed conditions are not limited to the aboveconditions. Examples of the prescribed conditions also include the casein which the headway distance from the preceding vehicle is equal to orless than a prescribed threshold (e.g., the first headway distancethreshold L*1), and the case in which the relative speed has becomeequal to or greater than a prescribed threshold in the approachingdirection. The drive force control flag Ft can be set to 0 when theoperator has indicated an intention to change lanes (e.g., when the turnsignal has been switched on), or when the lateral displacement of thepreceding vehicle with respect to the host vehicle is equal to orgreater than a prescribed threshold. The drive force control flag Ft canbe set to 1 when the headway maintenance assist system has becomeoperable (when an operating switch (not shown) has been turned on).However, when the accelerator pedal reaction force cannot be applied dueto a failure of the accelerator pedal actuator 80 or when the headwaymaintenance assist system is inoperable, the drive force control flag Ftis set to 0. When the response is in the affirmative in step S820, theprocess advances to step S830. When, on the other hand, the response isin the negative in step S820, the process advances to step S850.

In step S830, a drive force command value of the target drive force τ*tthat is outputted to the drive force controller 60 is calculated. Whenthe accelerator pedal position is low, i.e., when the accelerator pedal81 is depressed only slightly, it may be difficult for the driver tonotice that a reaction force has been applied to the accelerator pedal81. For example, when the vehicle speed is low, the accelerator pedalposition is also generally low, as shown by the solid line in FIG. 8. Inview of this situation, the relationship between the accelerator pedalposition and the drive force is changed in the following manner so thatthe driver more readily feels the reaction force on the acceleratorpedal, i.e., so that the accelerator pedal 81 is pressed until aprescribed accelerator pedal position at is reached.

The difference between the prescribed accelerator pedal position at andthe actual (detected) accelerator pedal position is calculated from therelationship between the vehicle speed and accelerator pedal positionbased on the speed V of the host vehicle. This difference is set as anoffset value α of the accelerator pedal position.

The offset value α is subtracted from the current accelerator pedalposition Acc, and the drive force corresponding to the accelerator pedalposition after the offset value α has been subtracted is set to be thetarget drive force τ*t. In other words, the relationship between thecurrent accelerator pedal position Ace and the drive force is changedfrom the ordinary correspondence relationship shown by the broken linein FIG. 9 to the corrected correspondence relationship shown by thesolid line. The generated drive force is thereby reduced with respect tothe current accelerator pedal position Acc. Therefore, the drive forcedesired by the driver can no longer be obtained unless the driverpresses considerably more than usual on the accelerator pedal 81. Forthis reason, the driver will press more forcefully on the acceleratorpedal 81.

The curve showing the changed correspondence relationship shown by thesolid line in FIG. 9 matches the curve showing the ordinarycorrespondence relationship shown by the broken line when theaccelerator pedal position is increased. Therefore, the same drive forceas usual can be obtained when the driver has considerably depressed onthe accelerator pedal 81, such as in the case of passing the precedingvehicle.

When the value obtained by subtracting the calculated offset value αfrom the current accelerator pedal position Acc is 0 or less, the targetdrive force τ*t is set to 0. Then the process advances to step S410 ofthe flowchart shown in FIG. 2 after the target drive force τ*t has beencalculated in step S830.

In step S850, the target drive force τ*t to be outputted to the driveforce controller 60 is calculated differently from step S830. In stepS850, the drive force corresponding to the current accelerator pedalposition Acc is set to be the target drive force τ*t. In other words,the relationship between the current accelerator pedal position Acc andthe drive force is set to be the ordinary correspondence relationshipshown by the broken line of FIG. 9. When the target drive force τ*t hasbeen calculated in step S850, the process advances to step S410 of theflowchart shown in FIG. 2.

According to the headway maintenance assist system of the illustratedembodiment, the following effects are obtained.

(1) When the drive force control flag Ft is set to 1 the relationshipbetween the current accelerator pedal position Acc and the drive forceis changed to a correspondence relationship that is different than anordinary correspondence relationship. The manner in which theaccelerator is manipulated by the driver can thereby be influenced so asto establish an accelerator pedal position that allows the driver tomore readily feel the reaction force of the accelerator pedal.Therefore, the driver can be reliably alerted by applying a reactionforce to the accelerator pedal.

(2) The generated drive force is configured to decrease with respect tothe accelerator pedal position Acc. Therefore, when the drive forcecontrol flag Ft is set to 1, the drive force will decrease as long asthe accelerator pedal 81 is not depressed with a greater force. Sincethe driver can be urged to press the accelerator pedal 81 with greaterforce, the driver can be made to notice with greater certainty that areaction force has been applied to the accelerator pedal 81.

(3) The offset amount of the current accelerator pedal position Acc isset and the relationship between the current accelerator pedal positionAcc and the drive force is changed (corrected) by subtracting the offsetvalue α from the current accelerator pedal position Acc. The controlcontent can thereby be simplified and the reliability of control in thedriving/braking force controller 50 can be improved.

(4) When the value obtained by subtracting the offset value α from thecurrent accelerator pedal position Acc is 0 or less, the target driveforce τ*t is set to 0. Therefore, the drive force cannot be obtainedwhen the current accelerator pedal position Acc is lower than theaccelerator pedal position corresponding to the offset value α. Sincethe driver can thereby be urged to press the accelerator pedal 81 untilat least the accelerator pedal position corresponding to the offsetvalue α is reached, the driver can be made to notice with greatercertainty that a reaction force has been applied to the acceleratorpedal.

(5) An increase the accelerator pedal position causes the relationshipbetween the accelerator pedal position Acc and the drive force to assumethe ordinary correspondence relationship that existed before the offsetvalue α was subtracted from the current accelerator pedal position Acc.The same drive force as usual can thereby be obtained when the driverhas pressed on the accelerator pedal 81 with considerable force, as inthe case of passing a preceding vehicle. Therefore, the driver'sintention to accelerate can be given greater consideration.

The relationship between the current accelerator pedal position Acc andthe drive force is not limited to the description above. Therelationship can be set so that various characteristics such as thoseshown by the curves A to E of FIG. 10 are achieved, for example. When,for example, the correspondence relationship between the drive force andthe changed accelerator pedal position Acc in FIG. 9 described above isthe curve A, the drive force corresponding to the accelerator pedalposition can be set so as to be further reduced as shown by the curve B.Although the accelerator pedal 81 responds more slowly when operated bythe driver in this case, the acceleration characteristics of the hostvehicle are moderated.

The drive force can also be linearly output with respect to theaccelerator pedal position, as shown by the curve C. In this case, theacceleration characteristics are more apparent to the driver. When theaccelerator pedal position exceeds the accelerator pedal position Acc1,a match can be achieved with the curve that shows the ordinarycorrespondence relationship indicated by the broken line, as shown bycurve D. In this case, since a match is achieved with the curve thatshows the ordinary correspondence relationship indicated by the brokenline from an accelerator pedal position that is lower than in the caseof curve A, the acceleration characteristics can be made to more closelyreflect the acceleration intentions of the driver. The positions of thecurves shown in FIG. 10 are preferably modified from D to A, A to C, andC to B as the degree of approach between the host vehicle and precedingvehicle increases, i.e., as the headway distance is reduced, or as therelative speed with the preceding vehicle increases in the approachingdirection. Also, the curves preferably change in the same manner with areduction in the vehicle speed V. The drive force characteristics thatare in accordance with the vehicle speed and the degree of approach withthe preceding vehicle can be obtained by modifying curves in thismanner, and the driver can more readily notice the application ofreaction force on the accelerator pedal.

In the description above, the target drive force τ*t is set to 0 whenthe accelerator pedal position is less than the offset value α, but evenwhen the accelerator pedal position is less than the offset value α, asshown by the curve E indicated by the thin solid line, the target driveforce τ*t may be set to a value that is greater than 0, i.e., so thatsome drive force can be obtained. The curve may be modified in thedirection in which the target drive force is reduced with respect to theaccelerator pedal position as the degree of approach to the precedingvehicle increases in the same manner as described above with referenceto curve E. Specifically, as the vehicle speed is reduced, the curve Ecan be modified in the direction of the arrow (from E1 to E3) as thedegree of approach increases, as shown in FIG. 12.

The target drive force τ*ta is reduced (torque down) to τ*tb bymodifying the map showing the relationship between the accelerator pedalposition Acc and the drive force, but the rate of change (the rate atwhich the target drive force is changed) at that time may also bemodified in accordance with the vehicle speed, the degree of approach(relative speed and headway distance), and other factors, as shown inFIG. 12. Specifically, when the vehicle speed is low, the rate of changeof the target drive force is preferably increased in the case that thedegree of approach is considerable (the headway distance is low and therelative speed is high in the direction of approach).

After the relationship between the accelerator pedal position and thetarget drive force has been compensated, the curve is returned to itsoriginal state when the drive force control flag Ft has become 0 in stepS820 of FIG. 7. At this point, since the drive force will increase evenif the accelerator pedal position remains constant, the driver may becaused to feel uncomfortable when the torque is suddenly increased.Therefore, the torque is preferably increased at a speed that is lessthan the speed at which the torque is reduced in the manner describedabove. It is sometimes better to increase the torque at a lower rate ofchange in accordance with conditions. In cases in which, for example,the relationship between the accelerator pedal position and the driveforce is returned to its original state when the preceding vehicle is nolonger being detected by the laser radar 70, it is better to furtherreduce the rate of change of the torque as the headway distance to thepreceding vehicle becomes shorter, or as the relative speed in thedirection of approaching the preceding vehicle becomes greater. Whentraffic lanes are narrow, when the number of lanes of the road beingtraveled is few, and when a congested road is being traveled, thecharacteristics can be set so that the rate of change of the torque isreduced and acceleration is moderated in rain, snow, or other badweather, at night, or at other times. Such a configuration allows thedrive force to be changed without making the driver uncomfortable.

The present invention is not limited to the embodiments described above.In the illustrated embodiment, for example, the parameter Tr1 forcalculating the transient term L*r1 of the first headway distancethreshold was calculated using Equation 2 but an upper limit Ta_max canbe established for the calculated value to limit the maximum value, anda lower limit value can also be established to limit the minimum value.The maximum value can be set in accordance with the speed V of the hostvehicle, for example. FIG. 11 is a diagram showing an example of therelationship between the speed V of the host vehicle and the upper limitTa_max. In the same manner, in this modification, upper and lower limitvalues can be established for the parameter Tr2 for calculating thetransient item L*r2 of the second headway distance threshold.

Also, an upper limit α*1 _(max) can be assigned to the first targetdeceleration rate α*1, and an upper limit α*2 _(max) (α*2 _(max)>α*1_(max)) can be assigned to the second target deceleration rate α*2.

In the illustrated embodiment, the map used for the target drive forcewas changed, but the present invention is not limited to such aconfiguration. Rather than changing the map when the target drive forceis calculated from the detected value of the accelerator pedal position,the detected value of the accelerator pedal position can be corrected tocalculate a virtual accelerator pedal position, and the target driveforce can be calculated from the previous map based on the virtualaccelerator pedal position. When the map showing the relationshipbetween the accelerator pedal position and the target drive force is tobe changed, the corrected map must be calculated with considerationgiven to the state of the transmission and other factors, but suchcomputation is not required and the relationship between the acceleratorpedal position and the target drive force can be easily corrected aslong as the detected value itself of the accelerator pedal position iscorrected and the virtual accelerator pedal position is calculated.

The steady item L*h2 of the second headway distance threshold in theillustrated embodiment was calculated based on the speed V of the hostvehicle and the relative speed Vr, but the calculation can be made bymultiplying a prescribed time and the speed of the preceding vehicle, orcan be made based on at least one option selected from the speed of thehost vehicle, the relative speed, and the speed of the precedingvehicle.

In the above-described embodiments, brake fluid pressure was supplied tothe wheel cylinders to reduce the speed of the vehicle, but the vehiclecan be caused to decelerate by using engine braking, downshifting, andother types of deceleration control.

In the illustrated embodiment, the deceleration can also be performedusing engine braking when the headway distance L is less than the firstheadway distance threshold L*1, and the deceleration control can beperformed by supplying brake fluid pressure to the wheel cylinders 12,22, 32 and 42 when the headway distance is less than the second headwaydistance threshold L*2. In this case, the user can discern betweendeceleration control that is performed when the headway distance L isless than the first headway distance threshold L*1, and decelerationcontrol that is performed when the headway distance L is less than thesecond headway distance threshold L*2.

In the illustrated embodiment described above, the acceleration pedalreaction force control was performed for applying reaction force to theacceleration pedal 81 as long as the driver was operating theacceleration pedal 81 when the headway distance L between the hostvehicle and the preceding vehicle was less than the headway distancethreshold L*. Another possibility is to vibrate the acceleration pedal81 instead of applying reaction force to the acceleration pedal 81.Thus, the accelerator pedal actuator 80 can include a vibrationimparting device that constitutes a haptic information conveying sectionthat is configured and arranged to convey a risk potential to a driveras haptic information through the accelerator pedal 81.

In the illustrated embodiment, the larger target deceleration rateselected from the first target deceleration rate α*1 and the secondtarget deceleration rate α*2 was set as the final target decelerationrate to carry out deceleration control of the vehicle. However,deceleration control of the vehicle can be carried out after the firsttarget brake fluid pressure P*1 has been calculated based on the firsttarget deceleration rate α*1, the second target brake fluid pressure P*2has been calculated based on the second target deceleration rate α*2,and the larger of the two target brake fluid pressures has been set asthe final target brake fluid pressure.

In the illustrated embodiment, the difference between the prescribedaccelerator pedal position at and the accelerator pedal position wascalculated from the vehicle speed and the accelerator pedal positionbased on the host vehicle speed V, as shown in FIG. 13, and thisdifference was set to be the offset value α of the accelerator pedalposition, but the present invention is not limited by thisconfiguration. The prescribed accelerator pedal position αt can be aconstant value such as a position of 25%, and can be a value thatcorresponds to the headway distance L, the relative speed Vr, or anotherparameter.

The offset value α can be varied using a gear position of thetransmission, the engine speed, the slope of the road surface, or otherparameters. In the case that the offset value α is varied using a gearposition of the transmission, a higher offset value can be used for ahigher gear position, or an offset value αgr that corresponds to thegear position can be looked up in a map. In the case that the offsetvalue α is modified using the engine speed, the offset value α can beincreased as the engine speed is reduced, or an offset value α thatcorresponds to the engine speed can be looked up in a map. In the casethat the offset value α is modified using the slope (inclination) of theroad, the offset value α can be set higher on an uphill slope and loweron a downhill slope, whereby an unwanted situation can be prevented inwhich the host vehicle ceases to accelerate when the accelerator pedalposition is low on an uphill slope.

A suitable offset value α can be calculated in accordance with the stateof obstacles in the forward direction and the running condition of thehost vehicle by setting the prescribed accelerator pedal position at andoffset value α in such a manner. Since the relationship between theaccelerator pedal position Acc and the drive force can thereby bechanged in accordance with the state of obstacles in the forwarddirection and the running condition of the host vehicle, the driver canbe appropriately alerted by the application of accelerator reactionforce in accordance with the traveling environment.

In the headway maintenance assist system of the illustrated embodiment,the vehicle was caused to decelerate as long as the driver was notoperating the accelerator pedal 81 when the headway distance L betweenthe host vehicle and the preceding vehicle had become less than thefirst headway distance threshold L*1. However, the present invention isnot limited to this configuration. The headway maintenance assist systemof the illustrated embodiment can also be configured so thatdeceleration control does not occur even when the headway distance Lbetween the host vehicle and the preceding vehicle has become less thanthe first headway distance threshold L*1 under certain runningconditions.

When the drive force control flag Ft is set to 1 as described in theillustrated embodiment, a control routine whereby the relationshipbetween the accelerator pedal position Acc and the drive force ischanged to a correspondence relationship that is different from anordinary correspondence relationship can be applied to a variety ofapparatuses for alerting the driver by applying a reaction force to theaccelerator pedal.

In the headway maintenance assist system of the illustrated embodiment,the relationship between the output torque of the engine 6 and theaccelerator pedal position is changed by changing the relationshipbetween the accelerator pedal position and the target drive force τ*toutputted to the drive force controller 60, but the present invention isnot limited to this configuration, and the relationship betweenaccelerator pedal position and the drive force τt in the wheel shaftscan be changed, for example, by changing the gear ratio of an automatictransmission, changing the output torque of a motor (e.g., electricmotor) other than the engine 6, or by making other changes.

In the illustrated embodiment, the laser radar 70 basically correspondsto the preceding vehicle detection means. The accelerator pedal positionsensor 56 basically corresponds to the accelerator actuation amountdetection means. The accelerator pedal actuator 80 basically correspondsto the actuation reaction force generation means. The sensors 13, 23,33, 43, and 52 to 56 and the laser radar 70 basically correspond to therunning condition detecting means. The drive force controller 60basically corresponds to the drive source control means. Thedriving/braking force controller 50 basically corresponds to theaccelerator actuation reaction force calculation means, the drivingforce determination means, and the compensation means.

GENERAL INTERPRETATION OF TERMS

In understanding the scope of the present invention, the term“comprising” and its derivatives, as used herein, are intended to beopen ended terms that specify the presence of the stated features,elements, components, groups, integers, and/or steps, but do not excludethe presence of other unstated features, elements, components, groups,integers and/or steps. The foregoing also applies to words havingsimilar meanings such as the terms, “including”, “having” and theirderivatives. Also, the terms “part,” “section,” “portion,” “member” or“element” when used in the singular can have the dual meaning of asingle part or a plurality of parts. Also as used herein to describe theabove embodiment(s), the following directional terms “forward, rearward,above, downward, vertical, horizontal, below and transverse” as well asany other similar directional terms refer to those directions of avehicle equipped with the present invention. Accordingly, these terms,as utilized to describe the present invention should be interpretedrelative to a vehicle equipped with the present invention.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. For example, the size, shape, location ororientation of the various components can be changed as needed and/ordesired. Components that are shown directly connected or contacting eachother can have intermediate structures disposed between them. Thefunctions of one element can be performed by two, and vice versa. Thestructures and functions of one embodiment can be adopted in anotherembodiment. It is not necessary for all advantages to be present in aparticular embodiment at the same time. Every feature which is uniquefrom the prior art, alone or in combination with other features, alsoshould be considered a separate description of further inventions by theapplicant, including the structural and/or functional concepts embodiedby such feature(s). Thus, the foregoing descriptions of the embodimentsaccording to the present invention are provided for illustration only,and not for the purpose of limiting the invention as defined by theappended claims and their equivalents.

1. A vehicle headway maintenance assist system comprising: a precedingvehicle detection section configured to detect a headway distancebetween a host vehicle and a preceding vehicle; an accelerator actuationamount detection section configured to detect an accelerator actuationamount of an accelerator of the host vehicle; a reaction force computingsection configured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the change occurs at a time beforethe reaction force generating section generates the reaction force. 2.The vehicle headway maintenance assist system according to claim 1,wherein the compensation section is configured to change the first driveforce/accelerator actuation relationship so that the target drive forceis reduced with respect to the accelerator actuation amount in thesecond drive force/accelerator actuation relationship.
 3. The vehicleheadway maintenance assist system according to claim 1, wherein thecompensation section is configured to set an offset amount of theaccelerator actuation amount detected by the accelerator actuationamount detection section for changing the first drive force/acceleratoractuation relationship to the second drive force/accelerator actuationrelationship.
 4. The vehicle headway maintenance assist system accordingto claim 3, wherein the compensation section is configured to change thetarget drive force determined by the driving force determination sectionto zero when the accelerator actuation amount detected by theaccelerator actuation amount detection section is less than the offsetamount.
 5. The vehicle headway maintenance assist system according toclaim 3, wherein the compensation section is configured to change amagnitude of the offset amount in accordance with the running conditionof the host vehicle detected by the running condition detection section.6. The vehicle headway maintenance assist system according to claim 1,wherein the compensation section is further configured to prohibitchanging the first drive force/accelerator actuation relationship to thesecond drive force/accelerator actuation relationship when theaccelerator actuation amount detected by the accelerator actuationamount detection section has exceeded a prescribed value.
 7. The vehicleheadway maintenance assist system according to claim 1, wherein therunning condition detection section is further configured to calculate aapproach rate between the host vehicle and the preceding vehicle basedon the headway distance detected by the preceding vehicle detectionsection; and the compensation section is configured to change the firstdrive force/accelerator actuation relationship to the second driveforce/accelerator actuation relationship so that the target drive forcedecreases with respect to the accelerator actuation amount as theapproach rate increases.
 8. The vehicle headway maintenance assistsystem according to claim 1, wherein the running condition detectionsection is configured to detect at least one of a vehicle speed, arotational speed of an output shaft of the drive source, a state of atransmission for reducing and transmitting a speed of the output of thedrive source to a drive wheel, and a road surface inclination as therunning condition that is used by the compensation section.
 9. Thevehicle headway maintenance assist system according to claim 1, whereinthe compensation section is configured to change the target drive forcedetermined by the driving force determination section.
 10. The vehicleheadway maintenance assist system according to claim 1, wherein thecompensation section is configured to change the accelerator actuationamount detected by the accelerator actuation amount detection sectionand outputs a compensated accelerator actuation amount to be used by thedriving force determination section in determining the target driveforce.
 11. The vehicle headway maintenance assist system according toclaim 1, wherein the running condition detection section is furtherconfigured to determine whether the reaction force generating section isin an operable state as the running condition that is used by thecompensation section; and the compensation section is configured tochange the first drive force/accelerator actuation relationship to thesecond drive force/accelerator actuation relationship when the runningcondition detection section has determined that the reaction forcegenerating section is in an operable state.
 12. The vehicle headwaymaintenance assist system according to claim 1, wherein the runningcondition detection section is further configured to determine whetherthe host vehicle and the preceding vehicle are in a prescribedpositional relationship detected by the preceding vehicle detectionsection as the running condition that is used by the compensationsection; and the compensation section is further configured to changethe first drive force/accelerator actuation relationship to the seconddrive force/accelerator actuation relationship when the runningcondition detection section has determined that the host vehicle and thepreceding vehicle are in the prescribed positional relationship.
 13. Thevehicle headway maintenance assist system according to claim 1, whereinthe running condition detection section is further configured to detectat least one of a vehicle speed, a path of the preceding vehicle, and arelative speed between the host vehicle and the preceding vehicle as adetected condition that is used by the compensation section to return tothe first drive force/accelerator actuation relationship from the seconddrive force/accelerator actuation relationship; and the drive sourcecontrol section is configured to change a return rate of change of thetarget drive force when the compensation section returns to the firstdrive force/accelerator actuation relationship.
 14. The vehicle headwaymaintenance assist system according to claim 1, wherein the drive sourcecontrol section is configured to set a return rate of change of thetarget drive force when the compensation section returns to the firstdrive force/accelerator actuation relationship from the second driveforce/accelerator actuation relationship, with the return rate of changeof the target drive force being less than a compensating rate of changeof the target drive force that occurred when the compensation sectionchanged the first drive force/accelerator actuation relationship to thesecond drive force/accelerator actuation relationship.
 15. A vehicleheadway maintenance assist system comprising: preceding vehicledetection means for detecting a headway distance between a host vehicleand a preceding vehicle; accelerator actuation amount detection meansfor detecting an accelerator actuation amount of an accelerator of thehost vehicle; accelerator actuation reaction force calculation means forcalculating a reaction force to be generated in the accelerator based onthe headway distance that was detected; actuation reaction forcegeneration means for generating the reaction force that was calculatedin the accelerator; running condition detecting means for detecting arunning condition of the host vehicle; driving force determination meansfor determining a target driving force of the host vehicle in accordancewith the accelerator actuation amount that was detected; drive sourcecontrol means for controlling an output of a drive source towards thedriving force determined by the driving force determination section; andcompensation means for increasing a driver awareness of the reactionforce generated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcethat was determined and the accelerator actuation amount that wasdetected to a second drive force/accelerator actuation relationshipbased on the running condition detected by the running conditiondetection section; wherein the change occurs at a time before theactuation reaction force generation means generates the reaction force.16. A vehicle headway maintenance assistance method comprising:detecting a running condition of a host vehicle and an acceleratoractuation amount of an accelerator of the host vehicle; changing a firstdrive force/accelerator actuation relationship between a target driveforce of the host vehicle and the accelerator actuation amount that wasdetected to a second drive force/accelerator actuation relationshipbased on the running condition that was detected; controlling an outputof a drive source to approach the target drive force based on theaccelerator actuation amount that was detected using one of the firstand second drive force/accelerator actuation relationships dependingupon the running condition that was detected; detecting a headwaydistance between the host vehicle and a preceding vehicle; computing anactuation reaction force to be generated in the accelerator based on thedetected headway distance; and generating the calculated actuationreaction force in the accelerator, such that the changing changes thefirst drive force/accelerator actuation relationship to the second driveforce/accelerator actuation relationship at a time before the generatinggenerates the calculated actuation reaction force.
 17. A vehicle headwaymaintenance assist system comprising: a preceding vehicle detectionsection configured to detect a headway distance between a host vehicleand a preceding vehicle; an accelerator actuation amount detectionsection configured to detect an accelerator actuation amount of anaccelerator of the host vehicle; a reaction force computing sectionconfigured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the compensation section isconfigured to set an offset amount of the accelerator actuation amountdetected by the accelerator actuation amount detection section forchanging the first drive force/accelerator actuation relationship to thesecond drive force/accelerator actuation relationship.
 18. The vehicleheadway maintenance assist system according to claim 17, wherein thecompensation section is configured to change the target drive forcedetermined by the driving force determination section to zero when theaccelerator actuation amount detected by the accelerator actuationamount detection section is less than the offset amount.
 19. The vehicleheadway maintenance assist system according to claim 17, wherein thecompensation section is configured to change a magnitude of the offsetamount in accordance with the running condition of the host vehicledetected by the running condition detection section.
 20. A vehicleheadway maintenance assist system comprising: a preceding vehicledetection section configured to detect a headway distance between a hostvehicle and a preceding vehicle; an accelerator actuation amountdetection section configured to detect an accelerator actuation amountof an accelerator of the host vehicle; a reaction force computingsection configured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the compensation section is furtherconfigured to prohibit changing the first drive force/acceleratoractuation relationship to the second drive force/accelerator actuationrelationship when the accelerator actuation amount detected by theaccelerator actuation amount detection section has exceeded a prescribedvalue.
 21. A vehicle headway maintenance assist system comprising: apreceding vehicle detection section configured to detect a headwaydistance between a host vehicle and a preceding vehicle; an acceleratoractuation amount detection section configured to detect an acceleratoractuation amount of an accelerator of the host vehicle; a reaction forcecomputing section configured to calculate a reaction force to begenerated in the accelerator based on the headway distance detected bythe preceding vehicle detection section; a reaction force generatingsection configured to generate the reaction force calculated by thereaction force computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the compensation section isconfigured to change the accelerator actuation amount detected by theaccelerator actuation amount detection section and outputs a compensatedaccelerator actuation amount to be used by the driving forcedetermination section in determining the target drive force.
 22. Avehicle headway maintenance assist system comprising: a precedingvehicle detection section configured to detect a headway distancebetween a host vehicle and a preceding vehicle; an accelerator actuationamount detection section configured to detect an accelerator actuationamount of an accelerator of the host vehicle; a reaction force computingsection configured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the running condition detectionsection is further configured to determine whether the reaction forcegenerating section is in an operable state as the running condition thatis used by the compensation section; and the compensation section isconfigured to change the first drive force/accelerator actuationrelationship to the second drive force/accelerator actuationrelationship when the running condition detection section has determinedthat the reaction force generating section is in an operable state. 23.A vehicle headway maintenance assist system comprising: a precedingvehicle detection section configured to detect a headway distancebetween a host vehicle and a preceding vehicle; an accelerator actuationamount detection section configured to detect an accelerator actuationamount of an accelerator of the host vehicle; a reaction force computingsection configured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the running condition detectionsection is further configured to detect at least one of a vehicle speed,a path of the preceding vehicle, and a relative speed between the hostvehicle and the preceding vehicle as a detected condition that is usedby the compensation section to return to the first driveforce/accelerator actuation relationship from the second driveforce/accelerator actuation relationship; and the drive source controlsection is configured to change a return rate of change of the targetdrive force when the compensation section returns to the first driveforce/accelerator actuation relationship.
 24. A vehicle headwaymaintenance assist system comprising: a preceding vehicle detectionsection configured to detect a headway distance between a host vehicleand a preceding vehicle; an accelerator actuation amount detectionsection configured to detect an accelerator actuation amount of anaccelerator of the host vehicle; a reaction force computing sectionconfigured to calculate a reaction force to be generated in theaccelerator based on the headway distance detected by the precedingvehicle detection section; a reaction force generating sectionconfigured to generate the reaction force calculated by the reactionforce computing section in the accelerator; a running conditiondetection section configured to detect a running condition of the hostvehicle; a driving force determination section configured to determine atarget driving force of the host vehicle in accordance with theaccelerator actuation amount detected by the accelerator actuationamount detection section; a drive source control section configured tocontrol an output of a drive source towards the driving force determinedby the driving force determination section; and a compensation sectionconfigured to increase a driver awareness of the reaction forcegenerated in the accelerator by changing from a first driveforce/accelerator actuation relationship between the target drive forcedetermined by the driving force determination section and theaccelerator actuation amount detected by the accelerator actuationamount detection section to a second drive force/accelerator actuationrelationship based on the running condition detected by the runningcondition detection section; wherein the drive source control section isconfigured to set a return rate of change of the target drive force whenthe compensation section returns to the first drive force/acceleratoractuation relationship from the second drive force/accelerator actuationrelationship, with the return rate of change of the target drive forcebeing less than a compensating rate of change of the target drive forcethat occurred when the compensation section changed the first driveforce/accelerator actuation relationship to the second driveforce/accelerator actuation relationship.